you're his kid. When you were 8 when you've met ranpo. you've met yukichi when you were 10 and ranpo 14. With the time ranpo suspect that you're yukichi's kid and you found out that it's true.

Ranpo and you had a fight, not something serious but it'll be understood if you're angry.

Fukuzawa knocked on your bedroom door, concerned that you're upset because of rampo. He maybe looks cold and act cold but he's not cold at all, he can't say no.